A lithographic mask (mask) is used to pattern a radiation sensitive layer on a semiconductor substrate. Typically, the radiation sensitive layer is called a photoresist layer. As used in this specification, a reticle is defined as the combination of a base and a patterning material. Typically, the base comprises a quartz plate, which is transparent to the exposing radiation, and the patterning material lies on one side of the quartz plate and comprises chrome, which is opaque to the exposing radiation. A stepping field is the portion of the patterning material that is used to pattern the photoresist layer. The photoresist layer is sensitive to reticle defects, such as particles, for example. If a particle is present on a reticle within the stepping field when a photoresist layer is exposed using the reticle, the particle may cause a corresponding image to be formed in the patterned photoresist layer, which in turn may cause the device being fabricated to fail. If the stepping field of the reticle contains only one die, then all die on the semiconductor substrate will fail.
A pellicle is typically used to reduce the likelihood that particles migrate onto the stepping field of a reticle. As used in the specification, a mask is defined to include a reticle and a pellicle, and the pellicle is defined to include a pellicle frame (frame) and a pellicle membrane (membrane). FIG. 1 illustrates a prior art mask having a reticle 100 and a pellicle 110 that forms a covered or protected area 120 over the stepping field. The pellicle 110 includes a membrane 130 and a single wall frame 140 having a single wall.
Although the prior art pellicle 110 may reduce particles within the protected area 120, the number of particles within the protected area 120 is typically still too high. The particles include adhesive particles, which are particles that originate from an adhesive material used to attach the frame 140 to the reticle 100 or membrane 130, and foreign particles, which are particles that originate outside of the pellicle. Referring to FIG. 1, the pellicle frame 140 is attached to the reticle 100 with a first adhesive material (not shown) and the membrane 130 is attached to the pellicle frame 140 with a second adhesive material (not shown). Both adhesive materials typically outgas and generate adhesive particles that may migrate onto the stepping field within the covered area. The outgassing and generation of adhesive particles increases as the adhesive materials are exposed to air or radiation, including both ambient light and radiation during exposure of a photoresist layer.
The prior art mask may also have problems related to pressure changes. A mask may be manufactured near sea level and be used at elevations greater than about 5000 feet, or vice versa. Pressure changes may also occur when the pellicle 110 is cleaned by using a nitrogen gun to blow particles off the membrane 130. The nitrogen gun typically causes the membrane 130 to vibrate when cleaning. The change in pressure from either source may cause the membrane 130 to change shape that typically accelerates adhesive particle generation leading to a larger number of particles on the surface of the reticle 100 within the covered area 120.
In order to equalize the pressure between the protected area and the area outside of the pellicle and reticle, in the prior art a pathway is provided (pressure relief pathway) through which air may pass from the region of higher pressure to the region of lower pressure. For example, the prior art mask of FIG. 1 has a pressure relief system that includes a conduit 160 that maintains about the same pressure on both sides of the membrane. The conduit 160 is a cylindrical-shaped hole through the pellicle frame. Foreign particles can migrate along a relatively straight path (through conduit 160) into the covered area 120. A screw or plug (not shown) may be placed within the conduit 160 to prevent particle migration. If the mask is used at a plurality of elevations and the screw or plug is not removed, the membrane may change shape and cause adhesive particle generation. When the screw or plug is removed to equalize the pressure on both sides of the membrane 130, foreign particles may enter through the conduit 160. Some masks may include a particle filter (not shown) within or over the conduit 160, such as Gortex.TM. brand material, but these masks may be difficult to generate or maintain, may require another adhesive material to attach to attach the filter to the frame, and are not currently widely used. However, use of such a filter would not reduce the likelihood of adhesive particle migration onto the reticle from the adhesive material used to attach the membrane to the frame and the frame to the reticle.